Investment casting is a conventional industrial process which employs a disposable pattern that is used to produce a ceramic mold in which an article or part can be cast. The pattern is conventionally made by injecting a liquid pattern material, e.g., wax, a heat-softenable plastic and the like, into a pattern die. The pattern die is typically manufactured from a durable material, e.g., aluminum, steel and the like, by conventional machining processes. The pattern is removed from the die after the pattern material solidifies.
A refractory material, e.g., an aqueous ceramic slurry, is then built up around the pattern to invest the pattern therein. A mold typically is produced by heating the refractory material having the pattern invested therein to remove the pattern and fuse the refractory material. The details of investment casting process vary depending upon the type of metal to be cast in the mold, the type of core material and the like. The casting of ferrous alloys can be used to illustrate a conventional investment casting process.
The pattern is coated with successive layers of refractory material. Each layer is coated with fine ceramic sand and dried before the next layer is applied. Usually about 10 to 20 layers are utilized to invest the pattern in the refractory material. The invested pattern is then placed in an open ended container which is filled with a coarse slurry of ceramic back-up material which hardens. The container is then placed into a furnace or autoclave. The temperature of the furnace or autoclave is elevated to cause the refractory material to dry and then fuse. The pattern is removed, as by melting or burning out the material constituting the same, during the heating step. The resulting fused ceramic structure is the desired mold.
Removal of the pattern leaves a cavity in the container corresponding in shape and dimension to the final part. The cavity (and therefore the pattern) can be slightly larger than the final part to compensate for the shrinkage which takes place in the subsequent casting operation, or to allow for machining when desired. The mold is sometimes fired to burn out the last traces of pattern material and to fuse the refractory material before the cavity is filled with molten metal. This firing process proceeds slowly in a controlled cycle which can be in a time range of 12 to 18 hours to avoid cracking the mold.
The molten metal is introduced into the cavity of the mold and solidified by cooling to form a casting. After solidification, the mold is broken away to release the part.
U.S. Pat. No. 3,465,808 discloses an investment casting method which involves forming a shell mold using a pattern composed of a mixture of polystyrene and a water soluble ethylene oxide polymer and removing the pattern after formation of the shell mold by treatment with steam under pressure.
U.S. Pat. No. 3,857,435 describes a process of making investment castings using a water decomposable core material that includes a nitrogen containing organo-ring material selected from mono and fused 5 and/or 6 membered ring compounds having as a ring constituent at least one nitrogen atom per every two rings of the compound, water soluble substitution products of said compounds, polymers of said compounds, and polymers of said substitution products of said compounds. It is stated that preferred materials comprise a mixture of these materials with polyethylene glycol and effervescent agents.
United Kingdom Patent 1,342,599 relates to a method of manufacturing an investment casting core by forming, in the shape of an investment casting core, a mixture of a ceramic powder and an organic binder, and heating the shaped core to a temperature insufficient to sinter the ceramic powder but sufficient to remove 48% to 96% by weight of the binder and to carbonize the remainder of the binder thereby to form a non-thermoplastic body of ceramic powder bonded by carbonaceous residues, the binder being so selected as to decompose during said heating to leave a residue of 0.4 to 14% by weight of the total weight of ceramic powder and the carbonaceous residues.
WO Patent Application 90/01727 discloses a method of investment casting that utilizes a pattern produced by stereolithography. The pattern has a polymeric matrix that has intersticial spaces dispersed throughout. The spaces have therein a thermoplastic material that flows from the matrix upon heating during the investment casting process, softening the pattern to prevent the thermal expansion of the pattern from cracking or deforming the mold.
United Kingdom Patent 2,074,065 describes a core for use in investment casting which comprises a ceramic material and a binder such as sodium aluminate which is soluble in water and stable up to such temperatures that the core is refractory above 800.degree. C. Such a core is manufactured by forming the mix of the ceramic material and binder into a granulated semi-dry powder which is then pressed in a die from which it is removed and dried and then fired before use. Alternatively, the ceramic material and binder are mixed with an additional organic binder with the resultant mix being granulated and injected under pressure into a die. After removal from the die, the core is fired to burn out the organic binder. When such cores are used they can be leached out after casting with a hot solution of dilute boric acid or other weak acid which acts to neutralize any sodium hydroxide formed.
U.S. Pat. No. 4,108,931 relates to a system of making molds for investment casting in which the patterns are formed from, and the sprue form is coated with, polyethylene glycol having a molecular weight in the range of from approximately 1,300 to approximately 1,600 or its equivalent and having admixed therein as a filler powdered graphite, in which the graphite comprises in the range of from about 35 percent to 60 percent by weight of the pattern forming material. The patterns are molded to the desired shape, and to include a gate section, and they are adhered to the sprue form in spaced apart relation by fluidizing the gate section end to make a welded connection with the sprue form coating. The mold is then formed about the patterns that are adhered to the sprue form, leaving the pour cup end of the sprue form exposed, to which heat is subsequently applied to sufficiently fluidize the sprue form coating so that the sprue form can be drawn out of the mold thereby leaving the sprue opening. The sprue form coating remaining in the mold sprue and the patterns are then removed without heating the mold by leaching with water, utilizing mechanical and/or chemical agitating after which the mold may be heated sufficiently to dry same for use as an investment casting mold.
U.S. Pat. No. 4,556,528 discloses a method and apparatus for casting of fragile complex shapes by the formation of a two layer wax mold about a pattern of the shape to be cast. Thereafter, the pattern is removed from the mold and the outer layer of wax dissolved. The slip is then poured into the mold and allowed to solidify before the inner layer of the mold is completely removed from the cast article.
U.S. Pat. No. 4,763,715 describes the evaporative casting of molten metals to produce castings having smooth surfaces without any sign of carbon deposits thereon by using a terpolymer polycarbonate prepared from cyclopentene oxide or cyclohexene oxide, ethylene oxide, propylene oxide, or butylene oxide, and carbon dioxide in making patterns for the lost foam casting process.
U.S. Pat. No. 4,773,466 relates to the evaporative casting of molten metals to produce castings having smooth surfaces without any sign of carbon deposits thereon by using a copolymer polycarbonate prepared from cyclohexene oxide, cyclopentene oxide, cycloheptene oxide, or isobutylene oxide and carbon dioxide.